This invention relates to a heat transfer sheet, more particularly, a heat transfer sheet which is useful in a heat transfer system by use of a sublimable dye (heat migratable dye), excellent in releasability between the dye carrying layer (the dye carrying layer is a layer comprising a dye and a binder; hereinafter called merely as dye layer) and the image-receiving material, and can give a mono-color or full-color image with excellent image density.
As the method capable of giving excellent mono-color or full-color image simply and at high speed in place of general letter printing method or printing method of the prior art, the ink jet system or the heat transfer system have been developed. Among them, as the system capable of giving a full-color image comparable with color photography having excellent continuous tone gradation, the so called sublimation heat transfer system by use of a sublimable dye is superior.
As the heat transfer sheet to be used in the above sublimation type heat transfer system, one having a dye layer containing a sublimable dye formed on one surface of a substrate film such as polyester film, while on the other hand, having a heat-resistant layer provided on the other surface of the substrate film for prevention of sticking of the thermal head, has been generally used.
By superposing the dye layer surface of such a heat transfer sheet on an image-receiving material having an image-receiving layer comprising a polyester resin, etc. and heating imagewise the heat transfer sheet from the back surface thereof with a thermal head, the dye in the dye layer is migrated onto the image-receiving material to form a desired image.
In the heat transfer system as described above, since a very high speed heat transfer is demanded, heating with thermal head is effected for a very short time (msec unit), whereby a high temperature is required for the thermal head.
As the result, when the temperature of the thermal head is elevated, the binder forming the dye layer is softened and sticks to the image-receiving material, whereby there occurs the inconvenience that the heat transfer sheet and the image-receiving material are adhered together, further causing in an extreme case a problem that the dye layer is peeled off during peeling of them to be transferred as such onto the image-receiving material surface.
In the prior art, for avoiding the above problem, there has been proposed a technique to form a curable silicone film separately on the surface of the dye layer (for example, Japanese Laid-open Patent Publication No. 209195/1986). However, in this method, when a curable silicone composition is coated on the dye layer, the solvent component in said composition attacks the dye layer, whereby the problem occurs that the dye is liable to be precipitated on the surface. Also, it is technically difficult to form a curable silicone film with a uniform thickness on the dye layer surface, and coating irregularity is liable to occur inevitably, which may consequently be a factor to cause sensitivity irregularity or formation irregularity of the image.
On the other hand, in the prior art, it has been also proposed to incorporate various release agents such as silicone polymers comprising perfluoroalkylated ester straight or branched alkyl or aryl siloxane units of straight alkyl or polyethyleneoxides or waxes (for example Japanese Laid-open Patent Publication No. 208994/1987). However, according to the knowledge of the present inventor, these release agents are not also necessarily satisfactory in improving releasability during heat transfer.
Accordingly, an object of the present invention is to provide a heat transfer sheet which can give an image of excellent quality without causing such problems as mentioned above.